Understanding laser-driven shock wave propagation is useful for a number of practical applications, ranging from nuclear fusion experiments [M. Schirber, “For nuclear fusion, could two lasers be better than one?” Science 310, 1610-1611 (2005)] to synthesis of protective materials [J.-H. Lee, D. Veysset, J. Singer, M. Retsch, G. Saini, T. Pezeril, K. Nelson, and E. Thomas, “High strain rate deformation of layered nanocomposites,” Nature Communications 3 (2012)], in addition to basic studies on materials in extreme conditions. However, direct measurements on shock waves propagating in solid media are difficult due to the high pressure generated by the shock. In transparent media, optical imaging techniques can be used to image the shock front [T. Pezeril, G. Saini, D. Veysset, S. Kooi, P. Fidkowski, R. Radovitzky, and K. Nelson, “Direct visualization of laser-driven focusing shock waves,” Physical Review Letters 106 (2011)], from which shock wave speed and peak pressure can be determined. However, in opaque media, measurement techniques are limited. A pressure sensor must be sufficiently rigid to withstand the shock wave, which may generate pressures around 1010 Pa. It must also be sufficiently small to provide a fast response time and to avoid measurement error due to the curvature of the shock front. Events that generate shock waves also generate large amounts of electromagnetic energy, which can interfere with electronic sensors.
Dielectric sensors, such as those based on fiber optics, can provide immunity from electromagnetic interference, a sufficiently small sensor head capable of a fast response time and a solid sensor head capable of withstanding extremely high pressures in a solid structure. Such a measurement capability will enable improved understanding of shock wave propagation in solid media by determining material characteristics such as shock wave speed and its relationship to shock pressure as well as behavior of the shock wave at interfaces and boundaries.
There have been many reported demonstrations of fiber optic sensors for measurement of ultrasonics and shock waves in liquids and air. The fiber Bragg grating (FBG) strain sensor has been investigated for measurement of ultrasonics in water [see, e.g., N. E. Fisher, D. J. Webb, C. N. Pannell, D. A. Jackson, L. R. Gavrilov, J. W. Hand, L. Zhang, and I. Bennion, “Ultrasonic hydrophone based on short in-fiber Bragg gratings,” Applied Optics 37, 8120-8128 (1998), P. Fomitchov and S. Krishnaswamy, “Response of a fiber Bragg grating ultrasonic sensor,” Optical Engineering 42, 956-963 (2003), and G. Flockhart, M. McGuire, S. Pierce, G. Thursby, G. Stewart, G. Hayward, and B. Culshaw, “Direct monitoring of underwater ultrasonic transducers in the near field using fibre Bragg grating sensors,” Proceedings of SPIE—The International Society for Optical Engineering 7503 (2009)]. Fabry-Perot sensors based on an air-backed diaphragm formed on the tip of an optical fiber have been reported for measurement of blast driven shock waves in air [see, e.g., W. N. MacPherson, M. J. Gander, J. S. Barton, J. D. C. Jones, C. L. Owen, A. J. Watson, and R. M. Allen, “Blast-pressure measurement with a high-bandwidth fibre optic pressure sensor,” Measurement Science and Technology 11, 95-102 (2000), S. Watson, M. J. Gander, W. N. MacPherson, J. S. Barton, J. D. C. Jones, T. Klotzbuecher, T. Braune, J. Ott, and F. Schmitz, “Laser-machined fibers as fabry-perot pressure sensors,” Applied Optics 45, 5590-5596 (2006), S. Watson, W. N. MacPherson, J. S. Barton, J. D. C. Jones, A. Tyas, A. V. Pichugin, A. Hindle, W. Parkes, C. Dunare, and T. Stevenson, “Investigation of shock waves in explosive blasts using fibre optic pressure sensors,” Measurement Science and Technology 17, 1337-1342 (2006), and W. Parkes, V. Djakov, J. S. Barton, S. Watson, W. N. MacPherson, J. T. M. Stevenson, and C. C. Dunare, “Design and fabrication of dielectric diaphragm pressure sensors for applications to shock wave measurement in air,” Journal of Micromechanics and Microengineering 17, 1334-1342 (2007)]. A solid Fabry-Perot formed on the tip of a fiber has also been reported for measurement of ultrasonics in liquids [P. Morris, A. Hurrell, A. Shaw, E. Zhang, and P. Beard, “A fabry-perot fiber-optic ultrasonic hydrophone for the simultaneous measurement of temperature and acoustic pressure,” Journal of the Acoustical Society of America 125, 3611-3622 (2009)]. A fiber tip sensor based on measurement in the change in the Fresnel reflection at the fiber endface has been demonstrated [J. Staudenraus and W. Eisenmenger, “Fiberoptic probe hydrophone for ultrasonic and shock-wave measurements in water,” Ultrasonics 31, 267-273 (1993)]. This utilizes the dependence of the refractive index on pressure in water, which modulates the reflected intensity from the fiber endface. This technique has been improved by using a tapered gold coated fiber tip [R. G. Minasamudram, P. Arora, G. Gandhi, A. S. Dary-oush, M. A. El-Sherif, and P. A. Lewin, “Thin film metal coated fiber optic hydrophone probe,” Applied Optics 48, G77-G82 (2009)]. Another fiber tip sensor based on the measurement of the phase shift in the light reflected from a mirrored fiber end, that forms one arm of a Michelson interferometer, has also been demonstrated for measurements of shock waves in liquids [C. Koch, G. Ludwig, and W. Molkenstruck, “Calibration of a fiber tip ultrasonic sensor up to 50 MHz and the application to shock wave measurement,” Ultrasonic's 36, 721-725 (1998) and C. Koch and K.-V. Jenderka, “Measurement of sound field in cavitating media by an optical fibre-tip hydrophone,” Ultrasonics Sonochemistry 15, 502-509 (2008)].
Despite numerous demonstrations of shock wave measurement in air and liquids, there have been no demonstrations in solids using fiber optic pressure sensors.